Move unboxed dispatch decision into dispatcher (#28251)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28251
Before, the dispatch key for unboxed operators from native_functions.yaml was generated in codegen and passed to the c10 dispatcher.
Now, we generate it inside of the dispatcher, right next to where the same thing happens for boxed calls.
ghstack-source-id: 93085152
Test Plan: unit tests
Differential Revision: D17989951
fbshipit-source-id: b343d9650debc62bfcff84cf4d6bdaf9dacc9d16
